The fuel cell cogeneration system of high power generation efficiency and high overall efficiency is given attention as a distributed power plant using energy effectively.
Many of fuel cells, for example, commercially practical phosphoric fuel cells and polymer electrolyte fuel cells being under development generate power using hydrogen as a fuel. However, hydrogen is not secured as an infrastructure, and thus needs to be produced at a site where the system is placed.
One method of producing hydrogen is a steam reforming method. A reformed gas obtained by mixing a raw material such as a hydrocarbon based material such as natural gas, LPG, naphtha, gasoline or kerosene, an alcohol based material such as methanol, and ether with water, and by subjecting the mixture to a steam reforming reaction in a reforming unit provided with a reforming catalyst is used as a hydrogen source such as in a fuel cell. In the case of the polymer electrolyte fuel cell operating at a low temperature of 100° C. or lower, however, the Pt catalyst for use in the electrode of the fuel cell may be rendered poisonous by CO contained in the reformed gas. If the Pt catalyst is rendered poisonous, the reaction of hydrogen is hindered, and thus power generation efficiency of the fuel cell is significantly reduced. For this reason, CO should be removed to reduce the concentration of CO to 100 ppm or smaller, preferably 10 ppm or smaller, using the hydrogen purification apparatus.
In this steam reforming reaction, carbon monoxide is produced as a by-product, but because the carbon monoxide deteriorates the fuel cell electrode catalyst, the carbon monoxide should be removed to reduce its concentration to 100 ppm or smaller, preferably 10 ppm or smaller especially for the polymer electrolyte fuel cell.
Usually, for removing CO, CO and steam are made to undergo a shift reaction to convert them into carbon dioxide and hydrogen in a CO shifting unit with a CO shifting catalyst body placed after the reforming unit in the hydrogen purification apparatus, thereby reducing the concentration of CO to the level of several thousand ppm to 1% by volume.
Thereafter, oxygen is added thereto using a very small amount of air, and CO is removed by a CO selection oxide catalyst body to reduce the concentration of CO to the level of several ppm at which the fuel cell is no longer adversely affected. Here, the amount of oxygen to be added should be one to three times as much as the CO concentration for sufficiently removing CO, but at this time, hydrogen is also consumed in response to the amount of oxygen to be added. If the concentration of CO at the outlet of the shifting unit is high, the amount of oxygen to be added is also increased, and the amount of consumed hydrogen is increased, thus significantly reducing efficiency of the overall apparatus.
Therefore, it is necessary to reduce CO sufficiently in the CO shifting unit with the CO shifting catalyst body placed therein.
Traditionally, for the CO shifting catalyst, copper-zinc based catalysts, copper-chrome based catalysts and the like capable of being used at 150 to 300° C. are used as low temperature CO shifting catalysts, and iron-chrome based catalysts and the like functioning at 300° C. or higher are used as high temperature CO shifting catalysts. For these CO shifting catalysts, only the low temperature CO shifting catalyst was used, or the high temperature CO shifting catalyst and low temperature CO shifting catalyst were used in combination depending on the uses of chemical plants and hydrogen generators for fuel cells.
However, when the copper based low temperature CO shifting catalyst is used as a main catalyst, a very high catalytic activity is obtained, but the catalyst needs to be activated by carrying out reduction treatment before it is used. Further, because the catalyst generates heat during the activation treatment, it was required to treat the catalyst for a long time while adjusting the amount of supplied reducing gas, for example, in order to prevent the situation in which the temperature of the catalyst is increased to above its heat-resistant temperature. Also, because the once activated CO shifting catalyst may be reoxidized and deteriorated if oxygen is entrained at the time when the apparatus is stopped and so on, it required some measures for prevention of oxidation and so on. In addition, the low temperature CO shifting catalyst has poor heat resistance, and thus cannot be heated rapidly at the time of starting the apparatus, and therefore it required some measures of increasing temperature slowly and so on.
On the other hand, in the case where only the high temperature CO shifting catalyst is used, the situation in which the temperature rises to somewhat excessive level can be accepted because of high heat resistance, and therefore heating during startup or the like is easier.
However, the CO shift reaction is an equilibrium reaction that hardly proceeds toward reduction in CO concentration in the high temperature region, and it was difficult to reduce the CO concentration to 1% by volume or smaller when a high temperature CO shifting catalyst functioning only at high temperature was used. Therefore, there were cases where purification efficiency in a CO purification unit connected subsequently was reduced.
In this way, in conventional techniques, there have been problems in which they cannot be applied well to the application where the hydrogen purification apparatus is frequently started and stopped because it takes much time to start the apparatus, and because of complicated handling.
Also, as described in Japanese Patent Laid-Open No. 2000-178007, use of precious metal based catalysts not requiring reduction treatments has been proposed. The precious based catalyst is used at about 250 to 350° C., but in the high temperature region, not only carbon monoxide is reduced by the shift reaction, but also a reaction tends to occur in which carbon monoxide and hydrogen in the reformed gas is reacted with each other to produce methane as a by-product, and thus there have been cases where the efficiency of the apparatus is reduced.